Creating space for Sika adhesive on 7a tip-up
Thanks all for for the helpful discussion; I built the canopy aluminum structure according to plans, then read everthing I could find about canopies prior to making the first cut on the plexi. Reading the various threads helped make this a less stressful event.
I made the big cut today, and started fine tuning the canopy and rear windows, I need to paint the canopy frame before I can finish the install. So far, everthing looks good. Here is how I created room for the Sika adhesive:
I covered the three F631 channels and F725 canopy frame joggle with electrician's tape, followed with a layer of 1/8 by 1/2 inch closed cell foam self adhesive weather stripping from the local hardware store. (I used electrical tape first because all is temporary until I work up the ambition to paint, and the weatherstripping is difficult to remove from aluminum)
The 1/8th layer of self adhesive weather stripping gives the same desired thickness as the Sika bond. I used a light duty, 1" wide ratchet strap connected to the rear wing spar bolt holes on either side of the fusalage to suck the canopy down to mark the big cut, and also to fine tune the canopy after the big cut. I had to use 1" styrofoam spacers to push in the bottom of the canopy frame since the strap was at such an acute angle to the canopy.
After the frame is painted, the forward and aft F 631 channels will have 1/8 spacers and Sika adhesive as described earlier in this thread, and the middle F631 channel (rivetted to the aft channel) will have a layer of 1/8 self adhesive weatherstripping to help seal the tip-up canopy against air leaks. I will check the usual mail-order supply houses for an aircraft grade of self adhesive foam weather stripping, but I don't mind using the hardware store variety since it is a non-structural component.
Other than laying the weatherstripping on the parts mentioned above, the only extra work was to sand down the bottom inside lip of the canopy where it nests into the F725 joggle. I didn't remove the full 1/8 thickness along the length of the F725, less than 1/16 allowed things to line up nicely.
I plan on installing the windows with no screws, except for the F725 side rails; I will use screws as per plans while the adhesive is curing, and remove them afterwards.
FYI, I am Canadian, it is cold outside, and cool inside my shop. I used a 500 watt hot air blower to warm up the canopy for an hour prior to, and during cutting, and had great results. The cold scrap plexi cut-offs were quite brittle; I could get them to crack and break by flexing unsanded pieces.
Jay
RV7A tip up
Big chunks done, working on finishing kit